High voltage generator for electrostatic painting equipment

ABSTRACT

A generator is provided inside equipment, such as a sprayer, having a high voltage cascade including a voltage-gain transformer and a voltage multiplier arranged at the output of the transformer. The transformer is a double or multiple transformer, including at least two elementary transformers, whereof respective primary windings are mounted electrically in parallel, and whereof respective secondary windings are mounted electrically in series, the arrangement reducing a diameter of the cascade to facilitate incorporation in the sprayer or other equipment. The invention is applicable to painting robots for the automotive industry.

TECHNICAL FIELD OF THE INVENTION

The invention relates, in a general manner, to electrostatic paintingequipment, such as sprayers used on painting robots, in the automotiveindustry. More particularly, this invention concerns an assembly forgenerating high electrical voltage, capable of generating a high voltage(typically of several tens of kilovolts) applied to the electrode ofsuch electrostatic painting equipment, with the aim of electricallycharging the paint, so as to improve the efficiency of deposition ofthis painting equipment and the quality of paint application.

DESCRIPTION OF RELATED ART

Such devices are known, for example, through the documents U.S. Pat. No.3,608,823 A, U.S. Pat. No. 3,731,145 A and U.S. Pat. No. 5,067,434 A.

These devices generally consist of the combination of an electroniccontrol unit, generating an AC voltage of variable amplitude andvariable frequency, the so-called “low voltage”, and of a “cascade”comprising in particular a transformer and a voltage multiplier.

The electronic control unit delivers, in general, a substantiallysinusoidal, low AC voltage whose frequency is of the order of a few tensof kilohertz (in particular lying between 20 and 50 kHz) and whoseamplitude (peak-to-peak) is of the order of a few tens of volts.

The transformer customarily has a turns ratio of around 100, and itsoutput voltage is of the order of 10 000 volts (peak-to-peak). Themultiplier consists of rectifier stages each comprising a capacitor anda diode, two consecutive stages being mounted in series in such a way asto double the peak voltage emanating from the transformer.

The trend in the art of paint sprayers is moving toward, on the onehand, greatly reducing their bulkiness, and in particular their outsidediameter, so as to reduce the soiling caused by the excess paint sprayedin the painting booth.

However, on the other hand, the standards in force, in Europe and in theU.S.A., are currently making it necessary to integrate the high voltage“cascade” into the sprayers, so as to minimize the stored electricalenergy which is liable to be discharged in the event of sparkover. Moreprecisely, according to the currently applicable standards, the energyliberated during a sparkover must be less than 0.24 millijoules, forsolvent-based liquid paints, and be less than 5 millijoules for powderpaint, so as not to cause ignition. Hence, the energy in the highvoltage “cascade”, as well as in the sprayer itself, must be minimized.

These requirements have already led to the high voltage, “cascade” beingplaced inside the sprayer, given that, if this “cascade” is situatedoutside, it has to be linked to the sprayer by a high voltage cable,which is by nature capacitive, and which would store up too much energy.

The housing of the high voltage “cascades” inside the sprayers leads tothe reducing of their dimensions, and in particular of their crosssection. In order to maintain the appropriate insulation distances, forhigh voltages of the order of 100 kV, between the electrode of thesprayer and the “low voltage” side of the “cascade”, a minimum distance,of the order of some twenty centimeters, is necessary. The “cascade”must therefore be designed in such a way as to extend lengthwise alongthe sprayer. Stated otherwise, for a “cascade” of given volume, it isbetter for it to possess a greater length and a smaller cross section.

This requirement results also from the diameters of the sprayers beingever smaller, as already indicated above, these sprayers still having tohouse other components: valves, various ducts (air, solvent, paint),motor, turbine, device for measuring turbine speed (for example byoptical fiber).

The total length of the sprayers must however remain reasonable, sincethis length has an influence on the total width of the painting booths.

The cluttering of the voltage multiplier with a high voltage “cascade”,consisting of stages with capacitor and diode, may be reduced to themaximum extent by placing the diodes as near as possible to thecapacitors, and by reducing the diameters of the capacitors.

However, in dealing with capacitors whose dielectric consists of aceramic disk, the value of the capacitance of these capacitors is givenby the formula: C=∈×d/s, where:

-   -   ∈ is the dielectric constant of the ceramic,    -   d is the height of the ceramic disk,    -   s is the cross section of the ceramic disk.

By applying this formula, it is noted that, for a given dielectricconstant of the ceramic, a decrease in the cross section of the ceramicdisk leads to an increase in the value of the capacitance, and hence ofthe stored energy, this being detrimental to safety as already discussedabove. Moreover, it is necessary to use, for the capacitors, a ceramicwhose dielectric constant is as stable as possible as a function oftemperature, thus leaving a limited choice of materials, and hence ofthe factor ∈.

Additionally, the voltage multiplier is not, currently, the part of thehigh voltage “cascade” having the largest cross section, the transformercustomarily possessing a greater cross section. Thus, solely reducingthe diameter of the capacitors of the voltage multiplier is notsufficient to solve the problem posed here, and it would also benecessary to intervene on the transformer.

The transformer remains, however, the most difficult part to implement,and in particular to miniaturize, while retaining the power requiredhere, of the order of 15 VA, with a turns ratio of the order of 100.

The primary of the transformer is determined in such a way as to have aminimum magnetic induction of the order of 0.1 to 0.2 teslars, so as tominimize the iron losses, given by the formula: Piron (B)−B^((2+x))with: 0≦x≦1. This makes it necessary to have a large number N1 of turnsat the primary of the transformer, by applying the formula: B˜1/N1.Consequently, and in view of the desired turns ratio, the secondary ofthe transformer must comprise several thousand turns.

The turns of the primary are, in general, situated in the heart of thetransformer, while the turns of the secondary are arranged as severallayers, toward the outside. This involves large total transformer crosssections.

It is also advisable to minimize the overall losses in the transformer,which losses are made up in particular of the copper losses and of theiron losses.

The copper losses depend on the resistance of the primary winding and onthe resistance of the secondary winding. Each of these resistancesdepends on the total length of the winding and on the cross section ofthe wire which constitutes this winding. Thus, a smaller wire crosssection entails a larger resistance, for equal length, and consequentlyhigher copper losses. Likewise, an increase in the number of turnsincreases the total length of the winding, hence the copper losses.Simply decreasing the wire cross section, or increasing the number ofturns, do not therefore constitute satisfactory solutions per se.

As far as the iron losses are concerned, these depend on the magneticinduction, as already explained above, and also on the frequency and thenature of the magnetic material used to conduct the magnetic flux, aswell as the overall volume of this material. In order to operate at afrequency of a few tens of kilohertz, one uses ferrites, here chosenfrom among the various kinds and forms of ferrites conventionally usedin the field of electronics, to conduct the magnetic flux. In thisregard, likewise, the current solutions are not satisfactory; inparticular, a typical known implementation consists of a transformerwound on a cylindrical ferrite core, which does not close up themagnetic flux, so that the output voltage decreases as the currentincreases; the efficiency of deposition of the paint, which depends onthis output voltage, also diminishes with increasing current.

Similar problems, in particular of a dimensional nature, also arise inthe case of handheld guns for paint spraying.

SUMMARY OF INVENTION

The present invention aims to solve all the problems set forth above, byproposing a high voltage “cascade” implementation which is improved alsoin its “transformer” part, in such a way as to allow an appreciablereduction in diameter of the “cascade”, facilitating its incorporationinto the sprayer or other equipment, while retaining the power requiredand improving the operating conditions of the assembly.

For this purpose, the subject of the invention is essentially a highvoltage-generator for electrostatic painting equipment, said generatorcomprising in particular, housed inside the equipment, a high voltage“cascade” comprising on the one hand a voltage booster transformer, andon the other hand a voltage multiplier placed at the output of thetransformer, this generator being characterized in that said transformerconsists of a double or multiple transformer, made up of at least twoelementary transformers, whose respective primary windings are mountedelectrically in parallel, and whose respective secondary windings aremounted electrically in series, the series mounting of the secondarywindings providing an output voltage, the sum of the voltages across theterminals of each of the secondary windings, and which is the inputvoltage of the voltage multiplier.

For example, to implement the equivalent of a single transformer whoseturns ratio is equal to 100, the invention can be employed by combiningtwo elementary transformers, each of ratio equal to 50, the respectiveprimaries of the two elementary transformers being mounted in paralleland energized thus, both, by the low voltage, while the respectivesecondaries of these two elementary transformers, mounted in series,provide an appropriate output voltage (double the voltage across theterminals of each secondary) across the terminals of this seriesmounting, said output voltage being the input voltage of the voltagemultiplier.

The generator, which is the subject of the invention, is thuscharacterized by the particular combination of two or more elementarytransformers in the high voltage “cascade”. A preferred form ofimplementation of the invention provides for these two or moreelementary transformers to be arranged coaxially, following one another,in such a way that the maximum diameter or the maximum cross section ofthe “cascade” is not greater than the diameter or than the cross sectionof each elementary transformer.

Advantageously, each elementary transformer is a transformer ofcylindrical or substantially cylindrical outside shape, with apot-shaped ferrite part, ensuring magnetic flux closure.

Thus, for each of the elementary transformers, the number of turns ofthe secondary is divided by the number of elementary transformers (bycomparison with the current solution with a single transformer).Certainly, the invention leads to an increase in the number of primariesbut, all the primaries being mounted electrically in parallel, theelectric current flowing through them is divided, and it is thuspossible to implement them with a smaller wire cross section. Moreover,in an elementary transformer of smaller overall cross section, the turnsof the primary are less “buried” (situated at depth) and can benefitfrom more satisfactory thermal exchange. Additionally, the potentialdifference between turns, and between layers of turns, at the secondary,is smaller since the overall potential between the input and the outputof the secondary winding of each elementary transformer is divided bythe number of elementary transformers, equal for example to 2 or 3. Thismakes it possible to reduce the tolerances in respect of the leakagelines of the windings, and to fill in the window of the ferrite part inan optimal manner.

All these elements make it possible to reduce the cross section of theelementary transformers, hence the cross section of the “cascade”itself, insofar as this cross section is determined essentially by that,of the “transformer” part, and as the elementary transformers, forexample two or three in number, are preferably arranged coaxially.

According to a complementary characteristic of the invention, in orderthat the reduction in the dimensions of the high voltage “cascade” isnot accompanied by a disadvantageous decrease in the insulationdistance, provision is made for the “high voltage” side connection ofthe generator to be effected through an arrangement of baffles, makingit possible to lengthen the insulation distance of the high voltage withrespect to ground, without however increasing the geometrical length ofthe “cascade” and without requiring any additional item.

Overall, one thus obtains the high voltage “cascade” that can be readilyincorporated into a handheld paint gun or sprayer, even with a reduceddiameter, without this requiring an undesirable lengthening of thesprayer or gun.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood with the aid of the descriptionwhich follows, with reference to the appended schematic drawingrepresenting, by way of examples, a few embodiments of this high voltagegenerator for electrostatic painting equipment:

FIG. 1 represents, in perspective, a high voltage “cascade” of such agenerator, with its various constituent elements;

FIG. 2 is a circuit diagram of the transformer of the “cascade” of FIG.1;

FIG. 3 represents in perspective the same “cascade” molded in aninsulating resin;

FIG. 4 is a perspective view, simplified, of a paint sprayer into whichsuch a “cascade” is integrated;

FIG. 5 illustrates, in circuit diagram form, a variant of thetransformer.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 represents a high voltage “cascade”, with its constituents,arranged in line along one and the same central axis A, which aresuccessively: a “transformer” part 2, a voltage multiplier 3, a currentlimiting resistor 4, and a high voltage contact 5, all these componentsbeing connected together, mechanically and electrically. The assembly isenergized electrically, on the transformer 2 side, with an AC voltageprovided by an electronic control unit (not represented), this “lowvoltage” being brought to input terminals 6 and 7.

According to the invention, and in the example illustrated here in FIGS.1 and 2, the voltage booster transformer 2 is a double transformer,composed of two elementary transformers 2A and 2B, of identicalstructure, stationed side by side.

In detail, and as shown more particularly in FIG. 2, the firstelementary transformer 2A possesses a primary winding 8A and a secondarywinding 9A; the two windings 8A and 9A are coiled around the centralcore of a ferrite pot 10A, for example of the so-called “RM” type, whichallows the magnetic flux to close up. Similarly, the second elementarytransformer 2B possesses a primary winding 8B and a secondary winding9B; the two windings 8B and 9B are coiled around the central core of aferrite pot 10B, which allows the magnetic flux to close up.

From the two input terminals 6 and 7, the respective primary windings 8Aand 8B of the two elementary transformers 2A and 2B are mountedelectrically in parallel, so that each primary winding 8A or 8B isenergized under the same input voltage Ve, but is traversed by a currentof strength i/2 equal to half the strength i of the current delivered bythe control electronics.

On the other hand, the respective secondary windings 9A and 9B of thetwo elementary transformers 2A and 2B are mounted electrically inseries, so that the voltage across the terminals of each secondarywinding 9A or 9B is equal to half, i.e. Vs/2, the output voltage Vs ofthe transformer part 2.

This transformer part 2 possesses, typically, a turns ratio equal to100, that is to say its output voltage Vs is equal to 100 times itsinput voltage Ve. To achieve this result, it is advisable for eachelementary transformer 2A or 2B to possess a turns ratio equal to 50,this determining the ratio of the numbers of turns of the primary coils8A, 8B and secondary coils 9A, 9B of these elementary transformers 2Aand 2B.

The output voltage Vs of the transformer part 2 is again multiplied, bythe desired factor equal, for example to 20, in the voltage multiplier 3which, in a manner known per se, comprises a series of rectifier stageswith capacitors 11 and diodes 12, the voltage Vs constituting the inputvoltage of this voltage multiplier 3.

As shown in FIG. 3, the assembly of the previously described componentsis advantageously sunk into an embedding material 13, more particularlyinto an insulating resin. This embedding material 13 forms, in front ofthe “cascade”, that is to say in the region of the high voltage contact5, baffles 14 which make it possible to lengthen the insulation distanceof the high voltage, without thereby increasing the total constructionallength (along the axis A) of said “cascade”.

The high voltage “cascade”, implemented as has just been described, canbe incorporated into an electrostatic paint sprayer 15, in the mannerillustrated very schematically in FIG. 4, where this “cascade” isindicated, in its entirety, by the label 16. The cross section of thetwo elementary transformers 2A and 2B determines the maximum crosssection of the “cascade” 16, it being possible here for this crosssection to be maintained at a relatively small value. The sprayer 15 iscarried by a painting robot, not represented.

The number of elementary transformers, which make up the “transformer”part 2, may be greater than 2, this allowing an additional reduction inthe maximum cross section of the “cascade”. Thus, FIG. 5 illustrates avariant, in which this part 2 is made up of three elementarytransformers 2A, 2B and 2C. The respective primary windings 8A, 8B and8C of the three elementary transformers 2A, 2B and 2C are mounted inparallel, hence each energized under the input voltage Ve. Therespective secondary windings 9A, 9B and 9C of these three elementarytransformers 2A, 2B and 2C are mounted in series, the voltage across theterminals of each of them being equal to a third (Vs/3) of the outputvoltage Vs.

The number of elementary transformers of the “cascade”, or theirmultiplication ratio, or else the number of stages of the voltagemultiplier, may naturally be modified without departing from the scopeof the invention. With the same idea in mind, the invention isapplicable not only to paint sprayers carried by painting robots, butalso to handheld paint guns, and to other similar equipment. Finally,the high voltage generator, which is the subject of the presentinvention, is equally applicable to equipment for powder paint as toequipment for liquid paint.

1. A high voltage generator for electrostatic painting equipmentcomprising: a high voltage cascade housed inside the equipment includinga voltage booster transformer and a voltage booster multiplier placed atan output of the transformer, wherein said transformer comprises atleast two elementary transformers having respective primary windingsmounted electrically in parallel and respective secondary windingsmounted electrically in series, the series mounting of the secondarywindings providing an output voltage, the sum of the voltages acrossterminals of each of the secondary windings, which is an input voltageof the voltage multiplier, and wherein the two or more elementarytransformers are arranged coaxially, following one another, in such away that a maximum diameter or a maximum cross section of the “cascade”is not greater than a diameter or a cross section of each elementarytransformer.
 2. The high voltage generator as claimed in claim 1,wherein each elementary transformer is a transformer of substantiallycylindrical outside shape, with a pot-shaped ferrite part, ensuringmagnetic flux closure.
 3. The high voltage generator as claimed in claim1, wherein a high voltage side connection of the generator is effectedthrough an arrangement of baffles, making it possible to lengthen aninsulation distance of the high voltage with respect to ground.
 4. Thehigh voltage generator as claimed in claim 1, wherein the paintingequipment is an electrostatic paint sprayer carried by a painting robot.